This invention relates generally to the shortstopping (termination) of the polymerization of a conjugated diene optionally mixed with a vinyl-substituted aromatic in an aqueous emulsion, free radical polymerization process conducted at from -20.degree. to 70.degree. C., using a redox initiator. More particularly, this invention relates to the termination of the aqueous free radical emulsion polymerization of butadiene, optionally mixed with styrene (e.g. ESBR).
The field of aqueous emulsion free radical polymerization constitutes a well established art. The systems now used were generally established in the 1940's and the 1950's.
In emulsion polymerization processes, the monomer is emulsified in a medium such as water with the aid of emulsifying agents such as soaps and synthetic emulsifiers. Initially the monomer is present in the form of emulsion droplets dispersed in the continuous aqueous phase. According to the present invention the emulsion droplets play only a minor role and the polymerization in emulsion systems occurs primarily in the aqueous phase and not in monomer droplets.
Considerable work published in the last decades has shown that the process of emulsion polymerization is more complicated and more subtle than previously believed and several theories have been developed in order to explain the experimental data, such as particle size, the conversion-time relationship, the dependence of particle size and molecular weight on conversion, and the influence of surfactant, initiator and monomer.
The chain reaction in free radical emulsion polymerization is initiated by radicals that are generated either by decomposition of peroxides or diazo compounds or by an oxidation-reduction reaction (a redox couple).
Early free radical emulsion polymerizations which were carried out at 50.degree. C. or higher (hot polymerization), reflected the temperature dependency of the formation rate of free radicals and therefore, of polymerizations.
A subsequent research led to the discovery of oxidation-reduction reactions capable of generating radicals in sufficient numbers for adequate polymerization rates at temperatures as low as -40.degree. C. (cold polymerization).
Commonly employed dissociative initiators are potassium peroxydisulphate (K.sub.2 S.sub.2 O.sub.8), benzoyl peroxide and azobisisobutyronitrile (AIBN), whereas cumene hydroperoxide and p-menthane hydroperoxide are oxidants for redox couples.
The combination of potassium peroxydisulphate with a mercaptan, such as dodecyl mercaptan, is used to polymerize e.g. butadiene and styrene. In hot recipes the mercaptan furnishes free radicals by its reaction with the peroxydisulphate, and it limits the molecular weight of the polymer by reacting with and terminating the growing polymer chain; the resulting thiyl radical initiates the growth of another chain. This use of mercaptan as a chain transfer agent or modifier is of great commercial importance in the manufacture of emulsion SBR and polybutadiene, since the mercaptan allows control of the toughness of the product, which otherwise may limit further processing.
The conversion rate to polymer at 50.degree. C. is 5-6%/hr. Polymerization is terminated at 70-75% conversion or less since higher conversions lead to polymers with inferior physical properties, presumably because of cross-linking in the latex particle to form microgel or highly branched structure.
A further improvement in SBR production could be attained by the use of more active radical initiating systems that permitted polymerization at 5.degree. C. with high rates of conversion. These cold SBR polymers produced at lower temperature, but stopped at 60% conversion were found to have superior properties as to those of hot SBR.
At 5.degree. C. a 60% conversion to polymer occurs in about 8-12 hours. In the persulphate hot recipe the mercaptan initiates and functions as a chain transfer agent, but in the cold redox recipes, the mercaptan is not essential to initiation. The main difference lies in the initiator systems.
The usually applied phosphates and ethylenediaminetetra acetic acid (EDTA) act as buffers and complex with ferrous ions, thereby maintaining a constant solubility and limiting its reactivity. This reaction is very rapid at 0.degree. C.; the emulsion components are in separate phase and the reaction occurs only at the interface. In many cold recipes auxiliary reducing agents, such as sulphoxylates are used as components of a redox cycle, i.e. the ferric ion is reduced to the ferrous state. Reducing sugars are no longer in widespread use because of their cost and susceptibility to bacterial attack during storage.
Although this initiator system is primarily used for so-called cold polymerizations, it will be appreciated that such system can also be used for higher temperature polymerizations.
For property control reasons, it is a well established practice, that the termination of the polymerization is effected by the addition of a shortstop, which reacts rapidly with radicals and oxidizing agents, thus destroying any remaining initiator and polymer free radicals and preventing the formation of new radicals.
The unreacted monomers are then removed: first the conjugated diene and more particularly butadiene by flash distillation at atmospheric then at reduced pressure; second the styrene usually by steam stripping in a column.
A dispersion of antioxidant is added to protect the product. The latex is optionally mixed with oil, and coagulated by the addition of dilute acid e.g. sulphuric acid and optionally a salt, e.g. aluminium sulphate or sodium chloride.
In particular, shortstopping compositions should meet the requirements as disclosed in e.g. D. C. Blackley "Emulsion Polymerization", Theory and Practice 1975, p. 406.
Such chemicals included for example mixtures of a water soluble sulphide, such as hydrogen sulphide, ammonium sulphide, or sulphides or hydrosulphides of alkali or alkaline earth metals and an organic compound possessing a quinonoid structure, such as quinone or an organic compound which may be oxidized to a quinonoid structure, such as hydroquinone; N-substituted dithiocarbamates; reaction products of alkylene polyamines with sulphur, containing presumably sulphides, disulphides, polysulphides and/or mixtures of these and other compounds; dialkylhydroxylamines, N,N'-dialkyl-N,N'-methylenebishydroxylamines, dinitrochlorobenzene, dihydroxydiphenyl sulphide and dinitrophenylbenzothiazyl sulphide. It will be appreciated that the efficiency of the selected shortstopping agent will be dependent on inter alia the type of the initiator to be used.
U.S. Pat. No. 2,469,017, herein incorporated by reference, discloses a process for the polymerization of butadiene-1,3 in aqueous emulsion in the presence of a peroxygen compound, comprising supplying to the emulsion, at a monomer conversion degree of 20 to 80%, an organic compound possessing the quinone structure and a water soluble inorganic sulphide of the class consisting of hydrogen sulphide, ammonium sulphide and alkalimetal sulphides and hydrosulphides, the said compound of quinonoid structure being supplied to the emulsion at least as soon as the said sulphide and in an amount from 0.001 to 0.05% by weight based on the amount of monomeric material originally present in the emulsion before polymerization and the said sulphide being supplied to the emulsion in an amount from 0.01 to 0.2% by weight based on the amount of monomeric material originally present in the emulsion whereby further polymerization of monomers is immediately terminated.
As suitable peroxygen compound were indicated hydrogen peroxide, an alkali metal or ammonium persulphate or perborate, and more particularly potassium persulphate, whereas as shortstopping co-component hydroquinone is preferably used. Especially from column 3, lines 19-21, it may be derived that when the sulphide is used without hydroquinone the polymerization proceeds to significantly higher conversion.
U.S. Pat. No. 2,602,078, herein incorporated by reference, discloses the use of N-substituted dithiocarbamates as effective shortstopping agents for emulsion polymerization reactions, resulting in the production of polymers with substantially improved properties which are substantially snow white.
Especially from column 1, lines 20-33 in relation to column 4, lines 62-68 of said patent, it may be derived that the employment of e.g. sodium sulphide and hydroquinone in emulsion polymerization should lead to difficulties and was not attractive.
More particularly from example II of U.S. Pat. No. 2,616,875, it is known that sodium thiosulphate and sodium sulphide respectively can not be regarded as active ingredients of the shortstopping agents obtained by reaction of alkylene polyamines with sulphur.
Moreover it was known from U.S. Pat. No. 2,662,876, that the polymerization in aqueous emulsion of a butadiene-1,3 optionally mixed with other copolymerizable monomeric materials, could be efficiently terminated by the addition to the emulsion at any desired stage of polymerization, of a combination of substances consisting of:
(1) a solution of an agent selected from the class consisting of sulphur, alkylthiuram polysulphides, alkyl xanthogen polysulphides and water-soluble inorganic polysulphides, and
(2) an aqueous solution of a water-soluble salt of dithiocarbamic acid, wherein substance (1) being added in amounts equivalent to from 0.01 to 0.10% by weight of sulphur and substance (2) being added in amounts of from 0.01 to 0.10% by weight, based on the weight of monomer initially present in said emulsion. Both U.S. Pat. Nos. 2,616,875 and 2,662,876 are herein incorporated by reference.
As a preferred inorganic, water-soluble polysulphide, sodium polysulphide was proposed. However, especially from example 3 of U.S. Pat. No. 2,662,876 it is learnt that sodium sulphide alone as shortstopping agent was significantly less active as compared with the sulphur-carbamate stopping agent.
A shortstopping agent for an aqueous emulsion polymerization of butadiene-1,3 hydrocarbons and mixtures of butadiene-1,3 hydrocarbons and compounds, which contain a CH.sub.2 .dbd.CH group and are copolymerizable with butadiene-1,3 hydrocarbons, e.g. styrene, comprising a combination of alkali metal-hydrosulphite, -hydroxide and -nitrite, wherein the amount of added alkali metal hydrosulphite being from 0.1 to 1 part by weight per 100 parts by weight of polymerizable monomeric material originally present, the added alkali metal hydroxide being from 0.5 to 3 mols per mol of added alkali metal hydrosulphite and the added alkali metal nitrate being from 0.1 to 2 mols per mol of added alkali metal hydrosulphite, was known from e.g. U.S. Pat. No. 3,322,736.
U.S. Pat. No. 4,242,477, herein incorporated by reference, discloses an improved process for shortstopping the polymerization of butadiene and styrene in an emulsion, free radical polymerization process conducted at a temperature of from about 40.degree. C. to about 60.degree. C., using potassium or ammonium persulphate as polymerization initiator, comprising the use of ascorbic acid or the sodium or potassium salts thereof, in a quantity of ascorbic acid or salts thereof of from about 0.1 to about 0.75 parts by weight per 100 parts by weight of butadiene plus styrene charged in the polymerization system. Polymer produced does not discolour, does not have odour and appears not to contain chemicals detrimental to the use of the polymer in food- or drug-related applications. However, these shortstopping agents were indicated not to be used as shortstops in the cold redox or hot redox polymerization systems.
A preferred, more optimized, shortstopping agent for a redox initiation system which has been widely used on an industrial scale for many years, comprises a combination of dithiocarbamate and sodium polysulphide optionally mixed with sodium nitrite, the latter ingredient of which was regarded as useful for the prevention of undesired, uncontrolled polymerization of the non-converted monomers during the recovery of these monomers from the shortstopped polymerization reaction mixture.
This so-called wild polymerization will give rise to an increase in molecular weight of the final polymer and production of undesirable cross-linked polymer (popcorn polymer), which may lead to fouling of butadiene distillation and flashing equipment and even to bursting of pipes, as is disclosed in D. C. Blackley, "Emulsion Polymerization", Theory and Practice, 1975, p. 430-435.
However, a disadvantage of such combinations is that dithiocarbamates and nitrites are precursors for the formation of nitrosamines which are suspected to be carcinogenic.
It will be appreciated by a person skilled in the art, that according to the the prior art, sulphides were only used, if any at all, as one of the ingredients of shortstopping agents and certainly not as sole shortstopping agent. This teaching is also disclosed in D. C. Blackley, "Emulsion Polymerization", Theory and Practice, 1975, p. 406-417, and particularly p. 416, and from the publications referred to therein.
On the other hand efforts in present research and development in the rubber industry has greatly increased to prevent the formation of N-nitroso compounds from possible precursors in shortstopping agents during emulsion SBR manufacture. Guidance on this is provided in for example by Japanese patent applications Nos. 58,005,349; 58,005,350; 58,002,337 and 57,212,243 and Russian patent No. 971,850 which are herein incorporated by reference.